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conjunction at very nearly, but not exactly, the same point. Titan will have reached the point B, and Hyperion b. At a third conjunction the two will be a little above the line Bb, and so on. Really the conjunctions occur closer together than we have been able to draw them in the figure. In the course of nineteen years the point of conjunction will have slowly moved all round the circle, and the satellites will again be in conjunction at A.

Now the effect of this slow motion of the conjunction-point round the circle is that the orbit of Hyperion, or, more exactly, its longer axis, is carried round with the conjunction-point, so that the conjunctions always occur where the distance of the two orbits is greatest. The dotted line shows how the orbit of Hyperion is thus carried halfway round in nine years.

An interesting feature of this action is that it is, so far as we know, unique, there being no case like it elsewhere in the solar system. But there may be something quite similar in the mutual action of the first and third, and of the second and fourth satellites of Saturn on each other.

A yet more striking effect of the mutual attraction of the matter composing the rings and satellites is that, excepting the outer satellite of all, these bodies all keep exactly in the same plane. The effect of the sun's attraction, if there were nothing to counteract it, would be that in a few thousand years the orbits of these bodies would be drawn around into different planes, all having, however, the same inclination to the plane of the orbit